Integrated fluxgate magnetic gradient sensor

ABSTRACT

An integrated fluxgate magnetic gradient sensor includes a common mode sensitive fluxgate magnetometer and a differential mode sensitive fluxgate magnetometer. The common mode sensitive fluxgate magnetometer includes a first core adjacent to a second core. The first and second cores are wrapped by a first excitation wire coil configured to receive an excitation current that affects a differential mode magnetic field. The differential mode sensitive fluxgate magnetometer includes a third core adjacent to the first core and a fourth core adjacent to the second core. The third and fourth cores are wrapped by a second excitation wire coil configured to receive an excitation current that affects a common mode magnetic field.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication No. 62/166,798, filed May 27, 2015, titled “IntegratedFluxgate Magnetic Gradient Sensor,” which is hereby incorporated hereinby reference in its entirety.

BACKGROUND

Magnetic field measurement may be utilized in a variety of systems. Forexample, as current flows through a metallic material, a magnetic fieldis generated. Thus, the measurement of the magnetic field createdindicates the amount of current through the metallic material. However,in addition to the magnetic field generated by the current, backgroundmagnetic fields (e.g., the magnetic field created by Earth) may also bepresent. In order to account for background magnetic fields, themeasurement of a magnetic field gradient (i.e., the difference inmagnetic field density at one location and another location) is utilizedin many applications. For example, a magnetic field gradient may beutilized to measure industrial currents (e.g., currents through busbars). Conventional systems use two magnetic sensors, such as fluxgatesensors, to measure the magnetic field gradient of a magnetic field.

SUMMARY

The problems noted above are solved in large part by systems and methodsfor measuring a magnetic field gradient. In some embodiments, anintegrated fluxgate magnetic gradient sensor includes a common modesensitive fluxgate magnetometer and a differential mode sensitivefluxgate magnetometer. The common mode sensitive fluxgate magnetometerincludes a first core adjacent to a second core. The first and secondcores are wrapped by a first excitation wire coil configured to receivean excitation current that affects a differential mode magnetic field.The differential mode sensitive fluxgate magnetometer includes a thirdcore adjacent to the first core and a fourth core adjacent to the secondcore. The third and fourth cores are wrapped by a second excitation wirecoil configured to receive an excitation current that affects a commonmode magnetic field.

Another illustrative embodiment is a driver circuit that includes adifferential voltage driver and a single-ended voltage driver. Thedifferential voltage driver is configured to drive a differentialvoltage through a common mode sensitive fluxgate magnetometer and adifferential mode sensitive fluxgate magnetometer. The single-endedvoltage driver is configured to drive a single-ended voltage through thedifferential mode sensitive fluxgate magnetometer. An input to thedifferential voltage driver is a voltage across a common mode sense wirecoil which is included in the common mode sensitive fluxgatemagnetometer. An input to the single-ended voltage driver is a voltageacross a differential mode sense wire coil which is included in thedifferential mode sensitive fluxgate magnetometer.

Yet another illustrative embodiment is a method for measuring a magneticfield gradient. The method includes driving, by a differential voltagedriver, a differential voltage through a common mode compensation wirecoil wrapped around a first core and a second core. The method alsoincludes driving the differential voltage through a differential modecompensation wire coil wrapped around a third core and a fourth core.The method also includes driving, by a single-ended voltage driver, asingle-ended voltage through the differential mode compensation wirecoil. The method also includes sensing a magnetic field gradient voltageacross a shunt resistor that is coupled to the single-ended voltagedriver and the differential mode compensation wire coil.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of various examples, reference will now bemade to the accompanying drawings in which:

FIG. 1 shows an illustrative block diagram of a current measurementsystem utilizing an integrated fluxgate magnetic gradient sensor inaccordance with various embodiments;

FIG. 2 shows an illustrative block diagram of an integrated fluxgatemagnetic gradient sensor in accordance with various embodiments;

FIG. 3 shows an illustrative block diagram of a common mode sensitivemagnetometer and a differential mode sensitive magnetometer included inan integrated fluxgate magnetic gradient sensor in accordance withvarious embodiments;

FIG. 4 shows an illustrative block diagram of a driver circuit includedin an integrated fluxgate magnetic gradient sensor in accordance withvarious embodiments; and

FIG. 5 shows an illustrative flow diagram of a method for measuring amagnetic field gradient in accordance with various embodiments.

NOTATION AND NOMENCLATURE

Certain terms are used throughout the following description and claimsto refer to particular system components. As one skilled in the art willappreciate, companies may refer to a component by different names. Thisdocument does not intend to distinguish between components that differin name but not function. In the following discussion and in the claims,the terms “including” and “comprising” are used in an open-endedfashion, and thus should be interpreted to mean “including, but notlimited to . . . .” Also, the term “couple” or “couples” is intended tomean either an indirect or direct connection. Thus, if a first devicecouples to a second device, that connection may be through a directconnection, or through an indirect connection via other devices andconnections. The recitation “based on” is intended to mean “based atleast in part on.” Therefore, if X is based on Y, X may be based on Yand any number of other factors.

DETAILED DESCRIPTION

The following discussion is directed to various embodiments of theinvention. Although one or more of these embodiments may be preferred,the embodiments disclosed should not be interpreted, or otherwise used,as limiting the scope of the disclosure, including the claims. Inaddition, one skilled in the art will understand that the followingdescription has broad application, and the discussion of any embodimentis meant only to be exemplary of that embodiment, and not intended tointimate that the scope of the disclosure, including the claims, islimited to that embodiment.

In many systems, a magnetic field gradient is measured. For example, amagnetic field gradient may be utilized to determine the amount ofcurrent flowing through a bus bar. In conventional systems, two separatemagnetic sensors (e.g., fluxgate sensors) are utilized to measure themagnetic field gradient. For example, one sensor may be placed at onelocation within the magnetic field while the second sensor may be placedat a second location within the magnetic field. The output of the secondsensor then may be subtracted from the output of the first sensor. Thisrequires a precision subtraction of the sensor output signals. Thus,separate logic and/or additional analog components outside of thesensors is required to compute the subtraction and a relatively largeoffset is created. Additionally, as the magnetic field gets larger, morecurrent is required to compensate the fluxgate sensors. Thus, more poweris required in the system.

In some conventional systems, driver circuits are utilized to compensatethe fluxgate sensors. This compensation current, due to a feedback loopwith the fluxgate sensor, corresponds with the magnetic field at thelocation of the sensor and is typically sensed utilizing a shuntresistor. Because the conventional system requires two fluxgate sensors,two driver circuits are required to provide the compensation current tothe two fluxgate sensors, and two matched shunt resistors are requiredto sense the compensation current. Therefore, a conventional systemrequires matched shunt resistors as well as matched instrumentationamplifiers to read the voltage across the shunt resistors. This isdifficult to implement and requires excessive chip area and power use.Therefore, it is desirable to design a system that utilizes a singleintegrated magnetic gradient sensor and a driver circuit that does notrequire matched shunt resistors and matched amplifiers to drive thesensor.

In accordance with the disclosed principles, a single integratedfluxgate magnetic gradient sensor is disclosed. To compensate for commonmode magnetic fields, the integrated fluxgate magnetic gradient sensormay include a common mode sensitive fluxgate magnetometer surrounded bya differential mode sensitive fluxgate magnetometer. In the magneticdomain, common mode fields are magnetic fields in the same directionwhile differential mode fields are magnetic fields in oppositedirections. The common mode sensitive fluxgate magnetometer may includetwo cores adjacent to one another, each wrapped by three wires, acompensation wire, an excitation wire, and a sense wire. Thedifferential mode sensitive fluxgate magnetometer may also include twocores, one adjacent to one of the cores of the common mode sensitivefluxgate magnetometer and the second core adjacent to the second core ofthe common mode sensitive fluxgate magnetometer. Similar to the cores ofthe common mode sensitive fluxgate magnetometer, each of the cores ofthe differential mode sensitive fluxgate magnetometer is wrapped bythree wires, a compensation wire, an excitation wire, and a sense wire.However, the differential mode sensitive fluxgate magnetometer cores arewrapped by the wires such that the differential mode sensitive fluxgatemagnetometer is sensitive to the difference in the density of themagnetic field at the location of the two cores (i.e., sensitive tomagnetic fields in opposite directions in the two cores) while thecommon mode sensitive fluxgate magnetometer cores are wrapped by thewires such that it is sensitive to common mode fields at the location ofthe two cores (i.e., sensitive to magnetic fields in the same directionin the two cores).

The driver circuit is configured to compensate the cores of bothmagnetometers with a differential voltage and/or a differential modedrive current generated by a differential voltage driver. The sensedvoltage of the common mode sensitive fluxgate magnetometer is then fedback to the driver circuit to drive the differential mode drive current.Thus, the differential mode sensitive fluxgate magnetometer iscompensated for common mode magnetic fields. Additionally, the drivercircuit is configured to compensate the cores of the differential modesensitive magnetometer with a single-ended voltage and/or drive currentgenerated by a single-ended voltage driver. The sensed voltage of thedifferential mode sensitive fluxgate magnetometer is then fed back tothe driver circuit to drive the single-ended drive current. Once thesensed voltage of the differential mode sensitive fluxgate magnetometeris zero, the compensation current of the differential mode sensitivemagnetometer, as driven by the driver circuit, corresponds to thegradient of the magnetic field and may be measured as a voltage across asingle shunt resistor.

FIG. 1 shows an illustrative block diagram of a current measurementsystem 100 utilizing an integrated fluxgate magnetic gradient sensor 104in accordance with various embodiments. The current measurement system100 may include a bus bar 102 and the integrated fluxgate magneticsensor 104. Bus bar 102 may be a metallic bar or strip (e.g., aluminum,brass, and/or copper), that may have a cross-sectional area that isgreater than a wire, and is configured to conduct current. In someembodiments, bus bar 102 may be utilized within a battery bank, adistribution board, a substation, a switchboard, and/or any otherelectrical apparatus and/or system.

In some examples, bus bar 102 includes a hole of any shape (e.g.,circular), through the bus bar 102. Thus, as current 122 flows throughbus bar 102, which may be hundreds of Amperes in magnitude, a magneticfield 124 is generated around the hole. More particularly, magneticfield 124 may be generated as two field components, one around one halfof the bus bar 102 (one half of the bus bar 102 separated by the hole)and the other around the other half of the bus bar 102. Additionally,one or more background magnetic fields 126 (e.g., magnetic fieldgenerated by Earth) may surround the bus bar 102. The integratedfluxgate magnetic sensor 104 may be positioned within the hole in thebus bar 102 as a single integrated circuit. The integrated fluxgatemagnetic sensor 104 may be configured to measure the gradient of themagnetic field 124. In other words, integrated fluxgate magnetic sensor104 may be configured to determine the difference in magnitude of thedensity of magnetic field 124 in two separate locations. For example,integrated fluxgate magnetic sensor 104 may be configured to determinethe magnitude of the magnetic field 124 in one location within the holeand a separate location within the hole in bus bar 102. Moreparticularly, integrated fluxgate magnetic sensor 104 may be configuredto determine the total magnetic field (i.e., magnetic fields 124 and126) in the two locations. Because the background fields 126 are equalin both locations in the hole, the integrated fluxgate magnetic sensor104 may measure the gradient of the magnetic field 124 by determiningthe difference in the total magnetic field in the two locations.

The magnitude of the gradient of the magnetic field 124 then may beutilized to calculate and/or measure the magnitude of current 122. Insome embodiments, a processing device within the integrated fluxgatemagnetic sensor 104 may utilize the gradient of the magnetic field 124to calculate the current 122, while in other embodiments, integratedfluxgate magnetic sensor 104 may sense the gradient of the magneticfield 124 and transmit the gradient information to other devices forprocessing.

While the current measurement system 100 shown in FIG. 1 is configuredto measure current 122 through bus bar 102 utilizing integrated fluxgatemagnetic sensor 104, in alternative embodiments, current through anyother device may be determined by utilizing the gradient in a magneticfield measured by integrated fluxgate magnetic sensor 104. Furthermore,as discussed above, a current need not be determined by integratedfluxgate magnetic sensor 104. Instead, the integrated fluxgate magneticsensor 104 may be configured to sense the gradient in any magneticfield, including magnetic fields that are not located around bus bars,but in any location.

FIG. 2 shows an illustrative block diagram of integrated fluxgatemagnetic gradient sensor 104 in accordance with various embodiments. Theintegrated fluxgate magnetic sensor 104 may include a driver circuit202, a common mode sensitive magnetometer 204, a differential modesensitive magnetometer 206, and a processing device 208. In someembodiments, the components of the integrated fluxgate magnetic gradientsensor 104 (i.e., driver circuit 202, a common mode sensitivemagnetometer 204, a differential mode sensitive magnetometer 206, and aprocessing device 208) may be integrated on the same integrated circuitsubstrate and/or disposed in a common package. Driver circuit 202 may beconfigured to drive voltage and/or current to common mode sensitivemagnetometer 204 (i.e., drive a differential mode drive currentgenerated by a differential voltage driver to common mode sensitivemagnetometer 204) and differential mode sensitive magnetometer 206(i.e., drive a single-ended drive current generated by a single-endedvoltage driver to differential mode sensitive magnetometer 204). In someembodiments driver circuit 202 drives the common mode sensitivemagnetometer 204 through wire 222 and the differential mode sensitivemagnetometer through the wire 224. Additionally, the driver circuit 202may be coupled to common mode sensitive magnetometer 204 in a closedloop system, such that a sense voltage 226 from the common modesensitive magnetometer 204 may be utilized as an input into the drivercircuit 202. Similarly, the driver circuit 202 may be coupled todifferential mode sensitive magnetometer 206 in a closed loop system,such that a sense voltage 228 from the differential mode sensitivemagnetometer 206 may be utilized as an input into the driver circuit202.

Common mode sensitive magnetometer 204 may be a fluxgate magnetometerthat is configured such that it receives an excitation current thatgenerates a differential magnetic field and thus, is capable of sensingcommon mode magnetic fields as common mode sense voltage 226.Differential mode sensitive magnetometer 206 may be a fluxgatemagnetometer that is configured such that it receives an excitationcurrent that generates a common mode magnetic field and thus, is capableof sensing differential mode magnetic fields as differential mode sensevoltage 228.

In some embodiments, integrated fluxgate magnetic sensor 104 includesprocessing device 208. In alternative embodiments, integrated fluxgatemagnetic sensor 104 does not include processing device 208. Processingdevice 208 may be any type of electrical processing device, such as amicroprocessor and/or a microcontroller or other electrical processingdevice, and may include a processor core, memory, and programmableinput/output peripherals. The memory may be in the form of flash,read-only memory, random access memory, or any other type of memory orcombination of types of memory. The components of the processing device208 may be implemented as a system on a chip (SoC) on a singleintegrated circuit with the other components of integrated fluxgatemagnetic sensor 104. In alternative embodiments, the processing device208 may be implemented across multiple integrated circuits.

FIG. 3 shows an illustrative block diagram of common mode sensitivemagnetometer 204 and differential mode sensitive magnetometer 206included in an integrated fluxgate magnetic gradient sensor 104 inaccordance with various embodiments. Common mode sensitive magnetometer204 may include two magnetic cores (sometimes called bars) 302 and 304adjacent to one another. In some embodiments, cores 302-304 arecomprised of a ferromagnetic material (e.g., a nickel-iron soft magneticalloy with high permeability). An excitation wire coil 310 (sometimescalled a primary coil) may be wound around cores 302-304. An excitationcurrent 352 then may be driven through excitation wire coil 310 by anexcitation circuit (not shown). The excitation current may be analternating current that causes the cores 302-304 to enter into a cycleof magnetic saturation and unsaturation. When in an unsaturated state,cores 302-304 are highly permeable (i.e., there is a strong linkagebetween the coils of excitation wire coil 310). However, when in asaturated state, cores 302-304 are weakly permeable (i.e., there is noor a weak linkage between the coils of excitation wire coil 310). Thepoint at which the cores 302-304 saturate depends on the combinedmagnetic field 124-126 at the location of the common mode sensitivemagnetometer 204.

The excitation coil wire 310 is configured and/or wound around cores302-304 such that the excitation current 352 generates oppositeexcitation magnetic fields 332-334 in the in the cores 302-304. In thepresence of an external magnetic field (e.g., magnetic field 124 and/or126), one of cores 302-304 may saturate sooner than the other of cores302-304. This may induce a signal in a sense wire coil 314 that has arelationship to the combined magnetic field 124-126. The sense wire coil314 may be configured and/or wound around cores 302-304 such that thevoltage induced in the sense wire coil 314 is proportional to the sum ofthe field change in cores 302-304. In other words, the excitationmagnetic fields 332-334 are differential mode such that they cancel eachother out in a common mode sense. Thus, the common mode sense voltage226 is the voltage across the sense coil wire 314.

The common mode sense voltage 226 may be provided as an input to thedriver circuit 202 to drive a current through common mode compensationwire coil 222. Common mode compensation wire coil 222 is configuredand/or wrapped around cores 302-304, in some embodiments in the samedirection as the sense coil wire 314 is wrapped around cores 302-304,creating compensation magnetic fields 336-338 in cores 302-304.Compensation magnetic fields 336-338 are common mode fields (i.e., inthe same direction in both cores 302-304) and, in some embodiments,equal in magnitude. This provides compensation (i.e., corrects) for anyexternal magnetic fields (e.g., fields 124-126). Through the feedbackloop, the driver circuit 202 is configured to drive current throughcompensation wire coil 222 until the common mode sense voltage 226 iszero. The amount of current required to drive the common mode sensevoltage 226 to zero corresponds to the magnitude of the combinedmagnetic fields 124-126.

Differential mode sensitive magnetometer 206 may include two magneticcores 306-308. In some embodiments, core 306 is adjacent to core 302 andcore 308 is adjacent to core 304. Thus, in some embodiments, the cores306-308 are separated by the common mode sensitive magnetometer 206. Insome embodiments, cores 306-308 are comprised of a ferromagneticmaterial (e.g., a nickel-iron soft magnetic alloy with highpermeability). Furthermore, the cores 302-308 may all be approximatelythe same thickness. An excitation wire coil 312 may be wound aroundcores 306-308. An excitation current 354 then may be driven throughexcitation wire coil 312 by an excitation circuit (not shown). Theexcitation current 354 may be an alternating current that causes thecores 306-308 to enter into a cycle of magnetic saturation andunsaturation. When in an unsaturated state, cores 306-308 are highlypermeable (i.e., there is a strong linkage between the coils ofexcitation wire coil 312). However, when in a saturated state, cores306-308 are weakly permeable (i.e., there is no or a weak linkagebetween the coils of excitation wire coil 312). The point at which thecores 306-308 saturate depends on the combined magnetic field 124-126 atthe respective locations of the cores 306-308.

The excitation coil wire 312 is configured and/or wound around cores306-308 such that the excitation current 354 generates excitationmagnetic fields 342-344 in the cores 306-308 in the same direction(i.e., common mode). In the presence of an external magnetic field(e.g., magnetic field 124 and/or 126), one of cores 306-308 may saturatesooner than the other of cores 306-308. This may induce a signal in asense wire coil 316 that has a relationship to the difference in thecombined magnetic field 124-126 at the location of core 306 and thecombined magnetic field 124-126 at the location of core 308. The sensewire coil 316 may be configured and/or wound around cores 306-308 suchthat the voltage induced in the sense wire coil 314 is proportional tothe difference of the field change in cores 306-308. In other words, theexcitation magnetic fields 342-344 are common mode such that the sensecoil wire 316 is sensitive to a differential field. Thus, thedifferential mode sense voltage 228 is the voltage across the sense coilwire 316 and it corresponds with the gradient of the magnetic field 124.

The differential mode sense voltage 228 may be provided as an input tothe driver circuit 202 to drive a current through differential modecompensation wire coil 224. In some embodiments, differential modecompensation wire coil 224 is configured to have the same resistance ascommon mode compensation wire coil 222. Differential mode compensationwire coil 224 is configured and/or wrapped around cores 306-308, in someembodiments in the same direction as the sense coil wire 316 is wrappedaround cores 306-308, creating compensation magnetic fields 346-348 incores 306-308. Differential magnetic fields 346-348 are differentialmode fields (i.e., the magnetic field in core 306 is opposite indirection as the magnetic field in core 308) and, in some embodiments,equal in magnitude. Through the feedback loop, the driver circuit 202 isconfigured to drive current through compensation wire coil 224 until thedifferential mode sense voltage 228 is zero. The amount of currentrequired to drive the differential mode sense voltage 228 to zerocorresponds to the magnitude of the gradient of the combined magneticfields 124-126, and thus corresponds with the magnitude of the gradientof magnetic field 124.

FIG. 4 shows an illustrative block diagram of driver circuit 202included in integrated fluxgate magnetic gradient sensor 104 inaccordance with various embodiments. The driver circuit 202 may includea differential voltage driver 402, a single-ended voltage driver 404,and a shunt resistor 406. Differential voltage driver 402 may be anamplifier configured to drive a differential voltage and/or adifferential mode compensation current, via common mode compensationwire coil 222, through common mode sensitive fluxgate magnetometer 204.Additionally, to compensate for the common mode magnetic field,differential voltage driver 402 may be configured to drive thedifferential voltage and/or differential mode compensation current, viadifferential mode compensation wire coil 224, through the differentialmode sensitive fluxgate magnetometer 206. Thus, the differential voltagedriver 402 may be coupled to the common mode sensitive fluxgatemagnetometer 204 and the differential mode sensitive fluxgatemagnetometer 206 in parallel at terminals 422-424. The difference incurrent at from terminal 422 to 424 may be the differential modecompensation current. Furthermore, as discussed previously, the commonmode sense voltage 226 may be utilized as an input to the differentialvoltage driver 402. Thus, differential voltage driver 402 may beconfigured to compensate for the common mode magnetic field bygenerating a differential voltage (in the electrical domain).

The single-ended voltage driver 404 may be coupled, through its output,to shunt resistor 406 and the differential mode sensitive fluxgatemagnetometer 206. The single-ended voltage driver 404 may be anamplifier configured to drive a single-ended voltage and/or acompensation current, via differential mode compensation wire coil 224,through differential mode sensitive fluxgate magnetometer 206.Furthermore, as discussed previously, the differential mode sensevoltage 228 may be utilized as an input to the differential mode driver404. As discussed previously, the single-ended voltage (i.e., the outputof single-ended voltage driver 404) corresponds with the gradient of themagnetic field 124 due to the nature of the integrated fluxgate magneticgradient sensor 104 and the feedback loop with the driver circuit 202,and more particularly differential mode driver 404. In other words,single-ended voltage driver 404 may be configured to compensate for thedifferential magnetic field by generating a single-ended voltage whichcreates a current in coil wire 224 that is common mode (i.e., the samedirection in cores 306-308) and equal in magnitude.

Thus, the voltage across shunt resistor 406, labelled as magnetic fieldgradient voltage 408, and/or the current through shunt resistor 406corresponds with the magnetic field 124. Therefore, the magnetic fieldgradient voltage 408 may be sensed and provided to a processing device,such as processing device 208 for further processing. In this manner, asingle sensor (i.e., integrated fluxgate magnetic gradient sensor 104)may determine the gradient of a magnetic field compensating for thepresence of a common mode field.

FIG. 5 shows an illustrative flow diagram of a method 500 for measuringa magnetic field gradient in accordance with various embodiments. Thoughdepicted sequentially as a matter of convenience, at least some of theactions shown can be performed in a different order and/or performed inparallel. Additionally, some embodiments may perform only some of theactions shown. In some embodiments, at least some of the operations ofthe method 500, as well as other operations described herein, can beperformed by integrated fluxgate magnetic gradient sensor 104 includingdriver circuit 202, common mode sensitive magnetometer 204, and/ordifferential mode sensitive magnetometer 206 and implemented by aprocessor executing instructions stored in a non-transitory computerreadable storage medium.

The method 500 begins in block 502 with driving a differential voltageand/or a differential mode compensation current through a common modecompensation wire coil. For example, a differential voltage and/ordifferential mode compensation current may be driven by differentialvoltage driver 402 through common mode compensation wire coil 222 whichmay be wrapped around cores 302-304. The method 500 continues in block504 and 508. In block 504, the method 500 continues with sensing acommon mode voltage across a common mode sense wire coil. For example,common mode sense voltage 226 may be sensed across the common mode sensewire coil 314. The differential voltage through common mode compensationwire coil 222 affects the magnetic fields 336-338 and the common modesense voltage 226. The method 500 continues in block 506 with inputtingthe common mode sense voltage into the differential voltage driver. Forexample, the common mode sense voltage 226 may be input into anamplifier that comprises the differential voltage driver 402. This typeof feedback loop enables the differential voltage driver 402, with asufficient drive voltage and/or current, to drive the common mode sensevoltage 226 to zero.

In block 508, the method 500 continues with driving the differentialvoltage through a differential mode compensation wire coil. For example,the differential voltage may be driven by differential voltage driver402 through differential mode compensation wire coil 224 which may bewrapped around cores 306-308. The method 500 continues in block 510 withdriving a single-ended voltage and/or compensation current through thedifferential mode compensation wire coil. For example, a single-endedvoltage may be driven by single-ended voltage driver 404 through thedifferential mode compensation wire coil 224. In block 512, the method500 continues with sensing a differential mode voltage across adifferential mode sense wire coil. For example, differential mode sensevoltage 228 may be sensed across the differential mode sense wire coil316. The single-ended voltage and/or compensation current throughdifferential mode compensation wire coil 224 affects the magnetic fields346-348 and the differential mode sense voltage 228. The method 500continues in block 514 with inputting the differential mode sensevoltage into the single-ended voltage driver. For example, thedifferential mode sense voltage 228 may be input into an amplifier thatcomprises the single-ended voltage driver 404. The method 500 continuesin blocks 510 and block 516. In block 510, the method 500 continues withagain driving a single-ended voltage and/or compensation current throughthe differential mode compensation wire coil. This type of feedback loopenables the single-ended voltage driver 404, with a sufficient drivecurrent, to drive the differential mode sense voltage 228 to zero. Inblock 516, the method continues with sensing the magnetic field gradientvoltage across a shunt resistor. For example, the voltage across shuntresistor 406 may be sensed. Once the differential mode sense voltage isdriven to zero, the single-ended voltage and/or the compensation currentcorresponds with the magnetic field gradient voltage 408.

The above discussion is meant to be illustrative of the principles andvarious embodiments of the present invention. Numerous variations andmodifications will become apparent to those skilled in the art once theabove disclosure is fully appreciated.

It is intended that the following claims be interpreted to embrace allsuch variations and modifications.

1. An integrated fluxgate magnetic gradient sensor, comprising: a commonmode sensitive fluxgate magnetometer including a first core adjacent toa second core, the first and second cores wrapped by a first excitationwire coil configured to receive an excitation current that affects adifferential mode magnetic field; and a differential mode sensitivefluxgate magnetometer including a third core adjacent to the first coreand a fourth core adjacent to the second core, the third and fourthcores wrapped by a second excitation wire coil configured to receive anexcitation current that affects a common mode magnetic field.
 2. Theintegrated fluxgate magnetic gradient sensor of claim 1, wherein thecommon mode sensitive fluxgate magnetometer further includes a commonmode sense wire coil configured to output a voltage proportional to acommon mode magnetic field.
 3. The integrated fluxgate magnetic gradientsensor of claim 2, wherein the voltage across the common mode sense wirecoil corresponds to a sum of field change in the first and second coresof the common mode sensitive fluxgate magnetometer.
 4. The integratedfluxgate magnetic gradient sensor of claim 1, wherein the differentialmode sensitive fluxgate magnetometer further includes a differentialmode sense wire coil configured to output a voltage proportional to adifferential mode magnetic field.
 5. The integrated fluxgate magneticgradient sensor of claim 4, wherein the voltage across the differentialmode sense wire coil corresponds to a difference in field changes in thethird and fourth cores of the differential mode sensitive fluxgatemagnetometer.
 6. The integrated fluxgate magnetic gradient sensor ofclaim 1, further comprising a differential voltage driver coupled to thecommon mode sensitive fluxgate magnetometer and the differential modesensitive fluxgate magnetometer and configured to drive a firstcompensation current through a first compensation wire coil wrappedaround the first core and the second core and a second compensation wirecoil wrapped around the third core and the fourth coil.
 7. Theintegrated fluxgate magnetic gradient sensor of claim 6, furthercomprising a single-ended voltage driver coupled to the differentialmode sensitive fluxgate magnetometer and configured to drive a secondcompensation current through the second compensation wire coil.
 8. Theintegrated fluxgate magnetic gradient sensor of claim 7, wherein thecommon mode sensitive fluxgate magnetometer further includes a commonmode sense wire coil and an input to the differential voltage driver isa voltage across the common mode sense wire coil.
 9. The integratedfluxgate magnetic gradient sensor of claim 7, wherein the differentialmode sensitive fluxgate magnetometer further includes a differentialmode sense wire coil and an input to the single-ended voltage driver isa voltage across the differential mode sense wire coil.
 10. Theintegrated fluxgate magnetic gradient sensor of claim 7, furthercomprising a shunt resistor coupled to the single-ended voltage driverand the second compensation wire coil.
 11. A driver circuit, comprising:a differential voltage driver configured to drive a differential voltagethrough a common mode sensitive fluxgate magnetometer and a differentialmode sensitive fluxgate magnetometer; and a single-ended voltage driverconfigured to drive a single-ended voltage through the differential modesensitive fluxgate magnetometer; wherein an input to the differentialvoltage driver is a voltage across a common mode sense wire coilincluded in the common mode sensitive fluxgate magnetometer and an inputto the single-ended voltage driver is a voltage across a differentialmode sense wire coil included in the differential mode sensitivefluxgate magnetometer.
 12. The driver circuit of claim 11, furthercomprising a shunt resistor coupled to an output of the single-endedvoltage driver and the differential mode sensitive fluxgatemagnetometer.
 13. The driver circuit of claim 11, wherein thedifferential voltage driver is configured to drive the differentialvoltage through the common mode sensitive fluxgate magnetometer bydriving the differential voltage through a compensation wire coilwrapped around a magnetic core included in the common mode sensitivefluxgate magnetometer.
 14. The driver circuit of claim 11, wherein thesingle-ended voltage driver is configured to drive the single-endedvoltage through the differential mode sensitive fluxgate magnetometer bydriving the single-ended voltage through a compensation wire coilwrapped around a magnetic core included in the differential modesensitive fluxgate magnetometer.
 15. The driver circuit of claim 11,wherein the differential voltage driver is coupled to the common modesensitive fluxgate magnetometer and the differential mode sensitivefluxgate magnetometer in parallel.
 16. A method of measuring a magneticfield gradient, comprising: driving, by a differential voltage driver, adifferential voltage through a common mode compensation wire coilwrapped around a first core and a second core; driving the differentialvoltage through a differential mode compensation wire coil wrappedaround a third core and a fourth core; driving, by a single-endedvoltage driver, a single-ended voltage through the differential modecompensation wire coil; and sensing a magnetic field gradient voltageacross a shunt resistor coupled to the single-ended voltage driver andthe differential mode compensation wire coil.
 17. The method of claim16, further comprising: sensing a common mode sense voltage across acommon mode sense wire coil wrapped around the first and second cores;inputting the common mode sense voltage into the differential voltagedriver.
 18. The method of claim 16, further comprising: sensing adifferential mode sense voltage across a common mode sense wire coilwrapped around the third and fourth cores; inputting the differentialmode sense voltage into the single-ended voltage driver.
 19. The methodof claim 16, wherein the first core is adjacent the second core, thethird core is adjacent the first core, and the fourth core is adjacentthe second core.
 20. The method of claim 16, wherein the first, second,third, and fourth cores comprise a ferromagnetic material.